Atmospheric Flight

Gravity and Atmosphere

Gravity is what keeps a planet's gaseous atmosphere from
spreading out into space away from the planet. If we compare the gravitational
pull of each planet in our solar system we would find them to be different.
This is because a planet's gravity is related to its mass. Usually the greater
a planet's mass, the greater the gravitational pull. Let's look at the Earth
and Jupiter.

Earth

Jupiter

Jupiter's mass is 318 times greater than Earth's mass. That means Jupiter
has a stronger gravitational pull than the Earth does. If an object on
Earth weighs 100 pounds, place that same object on Jupiter and it will
weigh 234 pounds. As the planet rotates on its axis it exerts a centrifugal
effect on the atmosphere. Let's say that you are wearing a pair of lightweight,
loose fitting sunglasses. If you spin around quickly in one place, these
sunglasses will fly off your nose and move outward away from your face.
This is a centrifugal effect. The air molecules around a planet do the
same thing, especially if they are lightweight. Molecules in a gas are
in constant motion zipping around and bouncing off each other. Lightweight
gases such as hydrogen and helium move faster than medium-weight gases
like nitrogen and oxygen. The heavy gases like carbon dioxide move at
a slower rate of speed than the other two. To visualize this choose this
link to an animation of molecular motion. A large planet like Jupiter with
a strong gravitational pull is able to hold the light gases even though
they move at high speeds. A small planet like Earth or Mars cannot hold
onto lightweight gases. The moon is so small that its gravity is not even
strong enough to hold onto the heavy gases like carbon dioxide, that means
it has no atmosphere.

The gravity on Earth, however, is strong enough to hold onto gases like
nitrogen and oxygen. So when the Earth was forming, the lightweight gases
of hydrogen and helium escaped into space, leaving behind the heavier
gases of nitrogen and oxygen. Mars has only one-third the gravity of Earth,
but rotates very close to the same speed as Earth. This means that when
Mars was forming it could not hold onto nitrogen and oxygen gases. This
left Mars with only the heavy gases in its atmosphere, mostly carbon dioxide.